The present invention relates to an autonomous space navigation system. In particular, the present invention relates to a method and apparatus for accurately determining in real time the location of a spacecraft using sequentially received positioning information.
The location of a spacecraft (or any other vehicle, for that matter) is often of critical importance. The location of the spacecraft, for example, may determine its ability to achieve scientific research goals and to communicate important information to Earth, or other spacecraft. Furthermore, the location of a spacecraft is required to determine the spacecraft trajectory and therefore its future location as well as to plan changes in the trajectory to send the spacecraft to a desired location.
In the past, the navigation information required to determine the location of a spacecraft has often been generated on the ground. Thus, for example, ground stations typically obtain ranging information using a ground to spacecraft communications link (and, for example, the signal time delay present in the information carried by the link). Ground processing of the navigation information, however, is time consuming and expensive due to the number of ground stations required and the volume of information to be processed.
Deep space missions, for example, would require significant amounts of Deep Space Network (DSN) time to provide accurate location information. As a particular example, the Space Interferometry Mission (SIM), slated for launch in 2004, includes an optical interferometer operating in an Earth trailing holocentric orbit. The DSN, which is a ground based phased array of radio telescopes currently run by NASA, requires approximately 18 hours of DSN time per week to meet accuracy requirements for the SIM. At approximately $1700 per hour for a 37 meter DSN antenna, over $5,000,000 is required to maintain accurate positioning information over the expected 5 year SIM mission.
Furthermore, even when ground stations accurately track the position of a spacecraft, the ground stations become a vital link in the spacecraft's operation. Thus, the ground stations become prime targets for attack during hostilities, including nuclear strikes, as well as more subtle sabotage. In fact, any ground based solution to spacecraft tracking is open to such attacks.
Another attempt at providing location information (primarily for ground based systems) includes the use of Global Positioning Systems (GPS). GPS satellites, located in medium earth orbit (MEO) direct their signals down toward the surface of the earth. The GPS satellites thereby provide range and range rate information in their signals (provided, for example, using propagation delay and Doppler shift characteristics of the transmitted signals).
A ground based receiver, able to gather GPS signals simultaneously from four GPS satellites, may determine its position using the range and range rate information as well as knowledge (provided in the GPS signal) of the particular GPS satellite that transmitted the signal and that GPS satellite's broadcast location. Any spacecraft, of course, that travels beyond MEO altitude (including a common GEO satellite), cannot rely on continually receiving four GPS signals simultaneously to determine its location. In other words, the GPS satellite orbits and antenna pointing is designed to provide simultaneous coverage of the Earth, not the infinity of space.
A need has long existed in the industry for an accurate autonomous spacecraft navigation and positioning system.